A current mirror is a well-known building block in analog circuit design. A current mirror may be used to provide an output current from an input current. Further, a current mirror may be used to provide an output current that is the same as the input current, or the current mirror may be ratioed.
FIG. 1 illustrates a typical current mirror, which consists of transistors M0 and M1. Transistor M0 is arranged in a diode configuration with its gate coupled to its drain. The input voltage (Vin) of the current mirror is equal to VgsM0. VgsM0 is given by VTH+ΔV, where VTH is the threshold voltage, and ΔV is the over-drive, which should typically be several hundred milliVolts for good matching betweens transistors M0 and M1. Typically, transistor M1 operates in saturation, so that overdrive voltage ΔV is needed so that transistor M0 is also in saturation for good matching between transistors M0 and M1.
For example, in a case where VTH=0.7V and ΔV=300 mV, Vin=VgsM0=1.0V.